Directional optical coupler

ABSTRACT

A directional optical coupler is shown and described, which includes an optical cell that is made of a first transparent conductor, a second transparent conductor and an electro-optical member interposed between the two transparent conductors, the electro-optical member having an index of refraction which can be varied only along one crystal axis by application of an electric field.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. patent application Ser. No. 10/189,951,filed Jul. 3, 2002 now U.S. Pat. No. 6,842,573 in the name of CurtisAlan Birnbach and entitled DIRECTIONAL OPTICAL COUPLER. This applicationclaims priority to U.S. Provisional Application 60/303,015, filed Jul.5, 2001, entitled METHOD AND APPARATUS FOR DEFLECTING AND DISPLACINGOPTICAL SIGNALS, and U.S. Provisional Application 60/305,575, filed Jul.16, 2001, entitled TUNNELING OPTICAL MATRIX SWITCHING TECHNIQUE ANDMETHOD OF MANUFACTURE, the disclosures of which are hereby incorporatedby reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to an optical coupler and morespecifically to a directional optical coupler that includes anelectro-optic member having an index of refraction that can be varied byapplication of an electric field.

Optical couplers are well known devices used to direct light from onelight source to a light receiving member. In a known variety of opticalcouplers a layer of material is used which changes its index ofrefraction upon application of an electric field. The change in theindex of refraction of the material so used in the prior art devicescauses the light that is transmitted therethrough to change direction.The material used in this variety of optical couplers is anisotropic andthus exhibits significant index of refraction changes in more than onecrystal axis.

A directional optical coupler in the present invention includes anoptical cell that is comprised of a pair of opposing opticallytransparent conductors and an index of refraction variable layerinterposed between the two optically transparent conductors. The indexof refraction variable used in an optical cell according to the presentinvention exhibits a significant index of refraction variation alongonly one crystal axis upon application of an electric field and anegligible or no change in the index of refraction in other crystalaxes. Thus, a light that is displaced by an optical cell according tothe present invention will not have a changed polarization vector.

A directional optical coupler according to the present invention mayinclude one optical cell or two or more optical cells as described abovedisposed on a suitable substrate such as a glass substrate.

A directional optical coupler according to the present invention may beused in conjunction with a number of different conventional opticaldevices such as reflective separation multipliers as part of amultiplexer/demutiplexer device or a scanner.

In addition, an optical cell according to the present invention may beinterposed between two planar waveguides in order to act as a switch tomake tunneling possible or to prevent tunneling between the two planarwaveguides so as to act as switch therebetween. Such an arrangement maythen be used in a tunneling optical matrix to allow for switching oflight between a plurality of planar waveguides.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 graphically shows the variation of the index of refraction uponapplication of an electric field along two crystal axes in a materialused in an optical cell according to the present invention.

FIG. 2 shows a cross-sectional view of a directional optical coupleraccording to a first embodiment of the present invention.

FIG. 3 shows a cross-sectional view of a directional optical coupleraccording to a second embodiment of the present invention.

FIG. 4 shows a directional optical coupler according to the firstembodiment of the present invention with a reflective separationmultiplier of a first variety.

FIG. 5 shows a directional optical coupler according to the firstembodiment of the present invention with a reflective separationmultiplier of a second variety.

FIG. 6 shows a directional optical coupler according to the firstembodiment of the present invention with a reflective separationmultiplier of a third variety.

FIG. 7 shows a cross-sectional view of a portion of a tunneling opticalmatrix according to prior art.

FIG. 8 shows a cross-sectional view of a portion of tunneling opticalmatrix in which an optical cell according to the present invention isinterposed between two planar waveguides.

FIG. 9 schematically shows a top view of a tunneling optical matrix inwhich optical cells according to the present invention are used tooptically link rows and columns of planar waveguides.

FIG. 10 schematically shows a top view of a tunneling optical matrix inwhich optical cells according to the present invention are used tooptically link rows and columns of planar waveguides and includes aplurality runners that connect the electrodes of the optical cells torespective external contacts.

DETAILED DESCRIPTION OF EMBODIMENTS

A directional coupler according to the present invention makes use of anelectro-optic material having an index of refraction which can be variedby the application of an electric field. FIG. 1 illustrates theelectro-optical behavior of such a material under the application of avoltage. As shown in FIG. 1, varying the applied voltage causes asubstantially linear change in the refractive index of the material inone crystal axis, e.g., the x crystal axis, while the application ofvoltage causes little change in the index of refraction of the materialin another crystal axis, e.g., the y crystal axis.

In the specific example that is shown by FIG. 1, the index of refractionin the x crystal axis changes substantially linearly between about 2.0to about 2.75 as the electric field changes between about −1000 kV/cm toabout 1000 kV/cm. However, the index of refraction in the y crystal axischanges only slightly from about 2.29 to about 2.32 for the same rangeof applied electric field. Thus, the change in the index of refractionin the x crystal axis is twenty five times that of the change in that ofthe y crystal axis. In other words, the change in the index ofrefraction in the y axis is 0.0015 per every 100 kV/cm; whereas, thechange in x crystal axis 0.0375 per every 100 kV/cm. Such a differencebetween the variation in the index of refraction makes it possible totreat the change in the index of refraction of one crystal axis asinsignificant or non-existent, particularly when the applied voltage islow. For example, a change of 100 kV/cm will only change the index ofrefraction in the y direction by 0.0015, which is very negligible; whilethe same voltage will increase the index of refraction along the xcrystal axis by 0.0375, which is comparatively significant. Thus, thereis a substantial change in the index of refraction in the x crystal axisof more than ten times that of the y crystal axis. This makes the changein the index of refraction in the y crystal axis comparativelynegligible.

Such an optical property may be found only in some materials. Forexample, in an electro-optic material such as Strontium Barium Niobate(SBN), the electro-optical property described above can be observedalong the R33 axis of the crystal. That is, along the R33 direction inSBN the refractive index can be substantially changed along one crystalaxis by application of an electric field, while the other crystal axesremain substantially unaffected. Because of the property described abovewith reference to FIG. 1, the polarization vector of light does notchange when an electric field is applied. More importantly, due to thenegligible or essentially non-existent change in the index of refractionof other crystal axes interaction of light with other crystal axes canbe minimized thus better control can be obtained over the operation ofthe optical device.

Referring now to FIG. 2, a directional optical coupler according to apreferred embodiment of the present invention includes: a planarsubstrate 20, which may be composed of glass, or silicon, or any othersuitable material; first transparent conductor 22, which is preferablycomposed of Indium-Tin oxide, or any other suitable material, disposedover planar substrate 20; index of refraction variable layer 24 thatexhibits a substantial variation in its index of refraction only for onecrystal axis upon application of an electric field and little or novariation in its index of refraction for other crystal axes; and secondtransparent conductor 26 composed preferably of the same material asthat of first transparent conductor 22 and disposed over index ofrefraction variable layer 24 opposite first transparent conductor 22.The combination of first transparent electrode 22, index of refractionvariable layer 24, and second transparent electrode 26 constitute anoptical cell according to the present invention which may be disposedand incorporated with other elements other than substrate 20.Preferably, index of refraction variable layer 24 is composed of SBNwhich is oriented in a direction that exhibits significant variation inits index of refraction only in one crystal axis. SBN is a particularlydesirable material in that it can be useful for electromagnetic wavesthat fall in the range 380 nanometers to 4 micrometers, which is a broadspectrum, thus allowing an optical cell according to the presentinvention to operate in a rather wide band. However, the invention maynot be limited to SBN and other suitable materials may be used toconstruct an optical cell according to the present invention, providedthat the material exhibits the variable index of refraction propertysubstantially along one crystal axis while it does not exhibit asignificant variable index of refraction along other axes. Othersuitable material may be found among ternary or quaternary opticalmaterials as well as other optical materials.

An optical cell according to the present invention is operated byapplying a bias voltage to first transparent conductor 22 and secondtransparent conductor 26. The application of a bias voltage creates anelectric field which will change the index of refraction of index ofrefraction variable layer 24 along one crystal axis, but will not have asubstantial effect on other crystal axes. Therefore, light that istransmitted through index of refraction variable layer 24 will changedirection, but will not have its polarization vector substantiallychanged. This may be specially true if the applied electric field is notgreat.

Referring back to FIG. 1, if the applied electric field is chosen sothat the index of refraction in one direction is great while the changein the index of refraction in the other direction is infinitesimal, verylittle change, if any, will be observed in the polarization vector. Forexample, if the electric field is changed from −400 kV/cm to −200 kV/cm,the index of refraction in the x crystal axis changes from 2.17 to 2.5,while comparatively very little, if any, change is observed in the ycrystal axis. Thus, by properly choosing the applied electric field, thechange in the index of refraction in one crystal axis can be drasticallyreduced or kept near zero, while the index of refraction of another axiscan be changed substantially.

Limiting the range of the applied electric field in order to minimize ordepress the change in the index of refraction of one of the crystalaxes, however, does not limit the capability of an optical cellaccording to the present invention in that, according to anotherembodiment of the present invention, if more displacement is desired,two or more optical cells can be stacked on top of one another in orderto provide the desired light displacement without incurring anysubstantial change in another crystal axis. FIG. 3, in which likenumerals identify like features according to the earlier descriptionprovided above, shows an optical coupler according to a secondembodiment of the present invention. The directional coupler shown byFIG. 3 includes two optical cells according to the present invention,one of which is disposed on top of the other. As explained above, eachone of the optical cells shown in FIG. 3 can be operated in a relativelynarrow electric field range, thereby exhibiting a relatively substantialchange in one crystal axis, while showing an infinitesimal change or nochange in another crystal axis. Thus, each optical cell can cause partof the desired displacement of light, without changing the polarizationvector of the light. Using the configuration shown in FIG. 3, two ormore optical cells can be stacked on top of one another to achievehigher cumulative displacements without any change in the polarizationvector.

A directional optical coupler according to the present invention canalso be combined with other optical elements to obtain many otheroptical devices. For example, as shown in FIG. 4, directional opticalcoupler according to the first embodiment of the present invention maybe used in conjunction with a reflective separation multiplier to obtaindeviations in the order of tens of degrees, thereby allowing an opticalcell according to the present invention to be integrated with fiberoptics and used in such applications as multiplexing/demultiplexing in,for example, telecommunication routers, whereby an optical signal can beselectively routed from input port to another port.

Referring to FIG. 4 specifically, a reflection separation multipliercomprising of first mirror 28, second mirror 30, and third mirror 32 canbe disposed near but spaced by an air gap from a directional opticalcoupler according to the present invention. First mirror 30 in thereflective separation multiplier is substantially flat and disposedopposite second mirror 30 and third mirror 32, which are alsosubstantially flat and angularly spaced from one another. Mirrors 28,30, 32 may be formed on facets of an optical device such as prism. Anormal ray 31 of light emerging from an optical cell according to thepresent invention is reflected from first mirror 28 onto second mirror30 and reflected therefrom at an angle. A displaced ray of light 33 thathas been displaced by application of an electric field is reflected fromfirst mirror 28 at a different point onto third mirror 32 and thenreflected therefrom at an angle different from that of normal ray 31.The angular difference 34 between normal ray 31 and displaced ray oflight 33 allows the two lights to be directed to two different points.Using such an arrangement, a multiplexer/demultiplexer may beconstructed by, for example, directing the normal ray 31 to one opticalfiber (not shown) and the displaced ray 33 to another optical fiber (notshown) to demultiplex or receiving light from one fiber and receivinglight from another fiber and directing the two into a common fiber (notshown) in a reverse order to multiplex.

Also using the arrangement shown in FIG. 4 a scanner may be devised. Thereflective separation multiplier shown in FIG. 4 is particularly usefulin fuzzy logic or thresholding applications or other applications inthat light may be reflected from a relatively large number of points onfirst mirror 28 to second mirror 30 and avoid reflection toward thirdmirror 32 where it may be angularly spaced relevant to another ray oflight. Thus, a margin of error can be assigned to the application of anelectric field to the optical cell according to the present, which inturn allows one, for example, to devise a thresholding type applicationwhereby the crossing of a threshold is indicated when light is reflectedoff of third mirror 32.

FIG. 5 shows another arrangement which may be used to construct amultiplexer/demultiplexer. Referring to FIG. 5, a reflective separationmultiplier, comprising of first mirror 28 and first curved mirror 36, isdisposed near and spaced by an air gap from an optical cell according tothe present invention. First mirror 28 and first curved mirror 36 may bemirrors on faces of an optical element such as a prism.

A normal ray 31 of light emerging from an optical cell according to thepresent invention is reflected from first mirror 28 onto first curvedmirror 36 and reflected therefrom at an angle. A displaced ray of light33 that has been displaced by application of an electric field isreflected from first mirror 28 at a different point onto first curvedmirror 36 and then reflected therefrom at an angle different from thatof normal ray 31. The angular difference 34 between a normal ray 31 anddisplaced ray of light 33 allows the two lights to be directed to twodifferent points. Using the arrangement shown in FIG. 5, amultiplexer/demultiplexer or a scanner may be constructed as describedabove with reference to the embodiment shown in FIG. 4.

FIG. 6 shows yet another arrangement which may be used to construct amultiplexer/demultiplexer. Referring to FIG. 6, a reflective separationmultiplier comprising of first curved mirror 36 and second curved mirror38, which is opposite first curved mirror 36, is disposed near andspaced by an air gap from an optical cell according to the invention.First curved mirror 36 and second curved mirror 38 may be mirror regionsthat are deposited on a surface of an optical element such as a prism.

A normal ray 31 of light emerging from an optical cell according to theinvention is reflected from second curved mirror 38 onto first curvedmirror 36 and reflected therefrom at an angle. A displaced ray 33 oflight that has been displaced by application of an electric field isreflected from second curved mirror 38 at a different point onto firstcurved mirror 36 and then reflected therefrom at an angle. The angulardifference 34 between normal ray 31 and displaced ray of light 33 allowsthe two lights to be directed to two different points. Using thearrangement shown in FIG. 6, a multiplexer/demultiplexer may beconstructed as described above with reference to the embodiment shown inFIG. 4.

A directional optical coupler according to the second embodiment asshown in FIG. 3 may be used in conjunction with a reflective selectionmultiplier according to the arrangements shown in FIGS. 4, 5 and 6instead of a directional optical coupler according to the firstembodiment as shown in FIG. 2 to obtain the same advantageous resultsdescribed above.

The reflective separation multipliers shown in the embodiments of FIGS.4, 5 and 6 can be Free Space devices, waveguides or made by a thin film.In the Free Space version the reflective separation multiplier consistsof three mirrored facets on an asymmetrical pentaprism with specificangular relationships and physical lengths. In the waveguide and thinfilm modes, the mirrors are deposited on facets. In the waveguide modethe mirror is deposited as a thin film directly on the facet. In thethin film version the thin film mirror is deposited on the surface of asolid prism. The prism in the preferred embodiment has three nonadjacent surfaces containing the critical angular relationship forcreating the reflective separation multiplier by total internalreflection (TIR). Preferably, the input and output port regions haveanti-reflection coatings.

In addition, output beam separation can be increased by utilizing one ormore reflective curved surfaces in lieu of plane facets on the prism.

It is also practical to fabricate the cell structure of FIGS. 2 and 3directly on the input face of the device. This can be visualized bylooking at FIGS. 4, 5, and 6 and eliminating the air gap and substrate.Further it is practical to add a second deflector to the output of thereflective separation multiplier increase the separation even furtherand provide independent control of the secondary beam separator.

A further enhancement can be obtained by constructing the prism out of asingle crystal of SBN and adding field electrodes. This creates a prismwith a variable index of refraction.

In cases where the input beam diameter is too large for proper deviceoperation, an input lens system is provided to reduce the beam diameter.This input lens is positioned between the input fiber and the first cellof a directional optical coupler of the present invention.

It should also be noted that if an optical cell according to the presentinvention is driven with bi-polar signals, it is possible to double theangle of deviation. This can be accomplished by changing the controlcircuitry.

An optical cell according to the present invention may also be used inan optical switching matrix. The switching in an optical switchingmatrix according to prior art is based on selectively controlled opticaltunneling between planar waveguides using phase coherent synchronouscoupling through the isolation layer which has defined switching regionsimplemented in an electro-optical material. A conventional arrangementwhich uses optical tunneling between a pair of planar waveguides isshown in FIG. 7. Referring to FIG. 7, there are disposed on a substrate2: a conventional tunneling optical coupler includes first planarwaveguides 40; second planar waveguide 42; and isolation layer 44disposed between first planar waveguide 40 and second planar waveguide42.

FIG. 8 shows an arrangement in which an optical cell according to thepresent invention is interposed between first planar waveguide 40 andsecond planar waveguide 42. First planar waveguide 40 is spaced fromsecond planar waveguide 42 close enough to allow for evanescent modecoupling. An optical cell according to the present invention replaces aportion of isolation layer 44 thus making it possible to selectivelyallow or disallow optical tunneling to occur between first planarwaveguide 40 and second planar waveguide 42. According to an aspect ofthe present invention, second planar waveguide 42 is buried in substrate20 just below the top surface of substrate 20 such that the top surfaceof second planar waveguide 42 is flush with the top surface of substrate20. The optical cell according to the present invention is disposed overa portion of second waveguide 42 such that its top surface issubstantially flush with the top surface of isolation layer 44.According to another aspect of the present invention first planarwaveguide 40 is formed above the surface in the form of a ridge. Thistype of arrangement is advantageous in that it allows for a costeffective manufacture of a three dimensional device. Preferably, via 46may be used to connect transparent conductor 22 to the top surface whereit may be connected to a control device (not shown) for selectiveapplication a voltage to vary the index of refraction of index ofrefraction variable layer 24 in order to selectively allow or disallowtunneling.

In order to describe the theory of operation of a tunneling opticalswitching matrix that employs an optical cell according to the presentinvention, it is necessary to establish some conventions. Referring toFIG. 7, each element has an index of refraction which can be denotedwith its respective numeral. Thus, the index of refraction of firstplanar waveguide 40, isolation layer 44, second planar waveguide 42 andsubstrate 20 can be represented by n₄₀, n₄₄, n₄₂ and n₂₀ respectively.The mathematical relationship between the indices of these layers isgiven by:n ₄₄ >n ₂₀ >n ₄₄where: n₂₀ and n₄₄ can be equaland: n₄₀=n₄₂

In the case where n₂₀ and n₄₄ are equal, the device is called aSymmetrical Waveguide. When n₂₀ and n₄₄ are unequal, the device iscalled an Asymmetrical Waveguide. The present invention can beincorporated in either device.

When two waveguides are brought into extremely close proximity couplingwill occur through phase coherent energy transfer (optical tunneling).The indices n₄₀ and n₄₂ in the guiding layers must be larger than n₄₄,and n₂₀, and the thickness of the confining layer must be small enoughthat the evanescent tails of the guided modes overlap. In order forenergy transfer to occur between the two guides, they must haveidentical propagation constants. Thus, the indices and the thicknessesof the waveguiding layers must be very carefully controlled to providematching propagation constants. In the case of other devices embodyingthe synchronous coupling principle such as the prism coupler, theinteraction length must be carefully chosen for optimum coupling. Thecondition for total transfer of energy between the waveguides isanalyzed by the theory of weakly coupled modes which provides that acomplete interchange of energy between phase-matched modes occurs if theinteraction length in the z direction satisfies the relation:kL=π/2where: k=coupling coefficient

-   -   L=interaction length

The present invention is based on the disruption of the delicate balancenecessary to allow tunneling by changing the index of refraction of theisolation layer 44. By choosing an electro-optical material thatexhibits a change in index of refraction under the application of anelectric field, and using it in the isolation layer 44 where two planarwaveguides cross over one another, a switching action will occur whenthe electric field is applied.

The switch can be made to operate in either “normally open” or “normallyclosed” modes of operation by choosing appropriate indices of refractionfor each of the layers to orchestrate the necessary conditions to eithersupport or deny coupling in the “rest” (no electric field applied)state.

The arrangement shown by FIG. 8 may be employed in a matrix (crossed)arrangement as shown in FIG. 9. Referring to FIG. 9, a plurality ofplanar waveguides 40 are arranged in rows disposed over a plurality ofwaveguides 42 that are arranged in columns. Waveguides 40 preferablycross over waveguides 42 at a ninety degree angle. An optical cell isdisposed between waveguides 40 and waveguides 42 at their respectivecrossing points 48, according to the arrangement shown in FIG. 8, sothat it may selectively optically link the two.

Referring to FIG. 10, in a preferred embodiment, a plurality ofconductive runners 50 may be provided to connect transparent conductors22, 26 to respective electrical contacts 52 for external connection to acontrol device (not shown) that is used to apply a voltage betweenassociated transparent conductors 22, 26.

In its preferred embodiment, an optical cell according to the inventionis manufactured using silicon or glass as a substrate and conventionalsemiconductor type CMOS processing. Photolithographic masking is used todefine the various regions during processing. In the preferredembodiment, the transparent conductors 22, 26 are typically Indium-Tinoxide (90% Indium Oxide: 10% Tin Oxide) for operation in the nearInfra-red and Visible bands. The Active Electro-Optic layer is anymaterial with appropriate optical characteristics that exhibits a changein index of refraction under the application of an electric field. Thewaveguides may be formed by any process that yields a waveguide ofsufficient quality.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. An optical coupler comprising: an optical cell, said optical cellincluding: a first optically transparent electrode; a second opticallytransparent electrode spaced from said first optically transparentelectrode; an electro-optic layer interposed between said firstoptically transparent electrode and said second optically transparentelectrode, said electro-optic layer having an index of refraction thatis substantially variable only along one crystal axis upon applicationof an electric field and negligible along other crystal axes for saidelectric field; and a reflective separation multiplier that includes alight entry plane, a light exit plane, a first light reflective surfaceand a second light reflective surface, said first light reflectivesurface facing said light entry plane and said second light reflectivesurface, and said second light reflective surface facing said light exitplane, enabling multiple reflections between said opposing first lightreflective surface and, said second light reflective surface whereby atleast two different beams of light that enter through said light entryplane can be angularly deviated with respect to one another when exitingthrough said light exit plane.
 2. An optical coupler according to claim1, wherein electric field is created by application of a voltage betweensaid first optically transparent electrode and said second opticallytransparent electrode.
 3. An optical coupler according to claim 1,wherein said electro-optic material comprises of Strontium BariumNiobate.
 4. An optical coupler according to claim 1, further comprisinga substrate, wherein said optical cell is disposed on said substrate. 5.An optical coupler according to claim 1, wherein said first opticallytransparent electrode and said second optically transparent electrodecomprise of Indium-Tin oxide.
 6. An optical coupler comprising: anoptical cell, said optical cell including: a first optically transparentelectrode; a second optically transparent electrode spaced from saidfirst optically transparent electrode; an electro-optic layer interposedbetween said first optically transparent electrode and said secondoptically transparent electrode, said electro-optic layer having anindex of refraction that is substantially variable only along onecrystal axis upon application of an electric field and negligible alongother crystal axes for said electric field; and a reflective separationmultiplier disposed near but spaced from said second opticallytransparent electrode which includes a first mirror and a second mirrordisposed opposite to said first mirror.
 7. An optical coupler accordingto claim 6, wherein said first mirror is flat and said second mirrorincludes a pair of flat mirrors.
 8. An optical coupler according toclaim 6, wherein said first mirror is a substantially flat mirror andsaid second mirror is a curved mirror.
 9. An optical coupler accordingto claim 6, wherein said first mirror is a curved mirror and said secondmirror is a curved mirror.
 10. An optical coupler according to claim 1,wherein an index of refraction of one of the crystal axes of saidelectro-optic material varies ten times more than that of its othercrystal axes.
 11. An optical coupler comprising: at least two opticalcells each capable of changing the direction of light, each optical cellincluding: a first optically transparent electrode; a second opticallytransparent electrode spaced from said first optically transparentelectrode; and an electro-optic layer interposed between said firstoptically transparent electrode and said second optically transparentelectrode, said electro-optic layer having an index of refraction thatis substantially variable only along one crystal axis upon applicationof an electric field and negligible along other crystal axes for saidelectric field; wherein one optical cell is disposed over the otheroptical cell; and a reflective separation multiplier, which is disposednear but spaced from a second optically transparent electrode of one ofsaid optical cells, and includes a first mirror and a second mirroropposite said first mirror.
 12. An optical coupler according to claim11, wherein said first mirror is substantially flat and said secondmirror includes a pair of angularly displaced, substantially flatmirrors.
 13. An optical coupler according to claim 11, wherein saidfirst mirror is a substantially flat mirror and said second mirror is acurved mirror.
 14. An optical coupler according to claim 11, whereinsaid first mirror is a curved mirror and said second mirror is a curvedmirror.